Preparation of Solid Superacid Catalysts

Home , Acid strength, Solid acid, Superacid

石油学会誌 Sekiyu Gakkaishi, 39, (3), 185-193 (1996) 185

[Review Paper] Preparation of Solid Superacid Catalysts

Kazushi ARATA

Dept. of Science, Hokkaido University of Education, Hachiman-cho, Hakodate, Hokkaido 040

(Received September 1, 1995)

Our recent works on preparation of solid superacid catalysts, sulfate-metal oxides and tungstate-, molybdate-, or borate-metal oxides, are reviewed. Superacids with an acid strength of up to Ho≦-16.04 are obtained by adsorbing sulfate ion onto amorphous oxides of Fe, Ti, Zr, Hf, Sn, and Si followed by

calcination in air at above 500℃; a superacid of Al2O3 is prepared from the crystallized oxide. Superacids by metal oxides with an acid strength of up to Ho≦-14.52 are synthesized in the same manner as those of the sulfate superacids by supporting ZrO2 with WO3 (13wt% W) or MoO3 (6wt% Mo): impregnation of amorphous ZrO2 with aqueous ammonium metatungstate or molybdic acid dissolved in

ammonia water, followed by evaporation of water and calcination in air at 800-850℃. Similar superacids are prepared by impregnation of amorphous oxides of Sn, Ti, and Fe with ammonium metatungstate and calcination in air, the maximum activity for reaction of isopentane being observed

with calcination at 1000℃ for the Sn base material and 700℃ for those of Ti and Fe; borate supported on zirconia also shows superacidity, though weak in strength.

1. Introduction their catalytic actions, sulfate-supported metal oxides and oxides such as tungsten and molyb- With reference to Gillespie's definition, an acid denum oxides supported on metal oxides; in that is stronger than Ho=-12, which corresponds particular, the article focusses on procedure for to the acid strength of 100% H2SO4, is known as a preparation of solid superacids. superacid1),2). Among a large number of solid acids, SiO2-Al2O3 bears strong acid sites on the 2. Superacids of Sulfated Metal Oxides surface and its highest acid strength has been determined, so far, to be Ho=-12, whose value is 2.1. Preparation of SO4/Fe2O3, TiO2, ZrO2 in the range of superacidity. In the field of cata- 2.1.1. Preparation of Zr(OH)48),9) lytic chemistry, solid acid systems stronger than Two hundred grams of ZrOCl2・8H2O (guaran- acidic oxides such as SiO2-Al2O3 and zeolites have teed grade, Nakarai Chemicals, Ltd.) are dissolved been developed recently, which are place in the into 2.5l of distilled water in a 5l beaker, and category of solid superacids. ammonia water (28%) is added by dropping into In the 1980s, we studied synthesis of solid the aqueous solution, while stirring, until pH8 of superacid catalysts with acid strengths of up to the solution is attained, measured with a slip of pH Ho≦-16.04 on the surfaces of oxides of Fe, Ti, Zr, test paper. The precipitated mixture is allowed to Hf, Sn, Si, and Al by strong coordination of sulfate stand for a day, after further stirring for 30min. ion. The superacids were satisfactorily active in a The aqueous portion is decanted away from the heterogeneous system for reactions which are precipitates, and fresh water is added, followed by generally catalyzed by strong acid, especially by stirring, settling down, and decanting away the superacids, such as SbF5-HF and SbF5-FSO3H. aqueous portion; washing of the precipitates by Following the same procedure as those of the decantation is repeated until the total amount of sulfated superacids, we have also synthesized water used is 60l, after which almost no chloride another type of superacid, not containing any ions are detected in the washing. The precipitates sulfate matter but consisting of metal oxides, are finally isolated by filtration and dried at 100℃ which can be used at high temperatures over for 24h. 500℃. The hydroxide is prepared in the manner We have published several reviews concerning described above, from ZrO(NO3)2・2H2O (Nakarai the superacids3)-7). This review summarizes our Chemicals, Ltd., extra pure reagent) as a starting recent work on syntheses of solid superacids and material. Since residual nitrate ions are thermal-

石油学会誌 Sekiyu Gakkaishi, Vol. 39, No. 3, 1996 186 ly decomposed away, thorough washing of the finally sealed in an ampoule, while being hot, precipitates with distilled water, as mentioned until use. The superacid catalysts thus prepared above, is not necessary; three times would suffice from ZrOCl2, ZrO(NO3)2, Ti[OCH(CH3)2]4, and for the decantation by washing, thus in this point TiCl4 are referred to as SO4/ZrO2-I, SO4/ZrO2-II, the preparation being made easier. SO4/TiO2-I, and SO4/TiO2-II, respectively; the 2.1.2. Preparation of H4TiO48),9) catalysts from thermal decomposition and hydrol- A 290ml volume of Ti[OCH(CH3)2]4 (Wako ysis of Fe(NO3)3 are referred to as SO4/Fe2O3-I and Pure Chemical Industries, Ltd.) is added into 2l -II, respectively. of distilled water while stirring in a 5l beaker, and The catalysts can be also obtained by treatment the white precipitates formed are dissolved by with ammonium sulfate, but the catalytic activity gradually adding 250ml of conc. HNO3, while is usually lower than that with sulfuric acid. As stirring. Ammonia water (28%), -300ml, is stated above, the catalyst is usually calcined in a added into the aqueous solution while stirring, Pyrex tube and sealed in an ampoule until use to until the solution attains pH8, followed by further avoid humidity. The appearance of the catalysts stirring for 30min, allowed to stand for a day, with the sulfate treatment differs greatly from that followed by washing the precipitates by decan- without the treatment. The former catalysts are tation of the 5l beaker twice, filtering, and finally finely powdered solids which coat the wall of a dryed at 100℃ for 24h. glass ampoule obscuring vision, whereas the latter Another method of preparing H4TiO4 is by is not. Thus, it can be confirmed whether or not hydrolyzing TiCl4 as follows: 80ml of TiCl4 superacidity has been generated. Coating the (Wako Pure Chemicals) are gradually added into wall of the glass ampoule is, however, not the case 2l of distilled water in a 5l beaker cooled by when the sample has adsorbed moisture from the ice water, forming large amounts of HCl gas. air. The catalysts obtained from isopropoxide of Ammonia water is added until pH8 (at room Ti and nitrates of Fe and Zr as starting materials are temperature) is attained, followed by the above high in activity and easy to prepare. procedures; the precipitates are washed thoroughly 2.1.5. Catalytic Activities by decantation using 60l of water until no chloride The SO4/Fe2O3 catalysts were examined in the ions are detected in the filtrate. The aqueous dehydration reaction of ethanol as shown in portion might become cloudy during washing, but Table 110); SO4/Fe2O3-III was prepared by the the white washing can be decanted away. hydrolysis of FeCl3 as a starting material, and 2.1.3. Preparation of Fe(OH)3 and Amorphous followed by the treatment and calcining at 500℃. Fe2O38),9) The SO4/Fe2O3-II and -III catalysts, treated with Five hundred grams of Fe(NO3)3・9H2O (Wako 0.05-0.5M H2SO4 or (NH4)2SO4, showed quite Pure Chemicals, guaranteed reagent) are dissolved high activities, much higher than that of SiO2- in 2l of water in a 5l beaker, followed by hydro- Al2O3, which is well known as one of the catalysts lyzing with ammonia water (-300ml used, pH 8), having highest surface acidity. washing, and drying as explained above. The The catalytic action for the reaction of butane decantation washing is performed until the liquid which is catalyzed by superacids was examined, portion becomes cloudy (7-8 times). and it was found that the present catalysts are active Amorphous Fe2O3 is prepared by thermally for the skeletal isomerization of butane to iso- decomposing Fe(NO3)3・9H2O at about 200℃ or butane, at room temperature or even at 0℃, as higher; brown fuming gas is generated by shown in Fig. 111). SiO2-Al2O3 was totally inac- decomposition of iron nitrate after fusion of the tive for the reaction. Acid strength of SiO2-Al2O3 nitrate, and solid iron oxides are obtained upon used was in the range of -12.70hydroxides or oxides on a filter paper, followed by treating with H2SO4 and calcining had and drying in air; the iron materials are again a tendency to be converted into iron sulfate; the powdered because of solidifying after drying. catalyst obtained by using 0.05M showed activity The materials are calcined in a Pyrex glass tube in higher than that using 0.25M for dehydration of air at temperatures above 500℃ (600-650℃ for Zr ethanol, as shown in Table 1, and only the catalyst samples, 525℃ for Ti, 500℃ for Fe) for 3h and prepared by treatment with 0.05M showed activity

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Table 1 Dehydration of Ethanol over SO4/Fe2O3 at 250℃

a) Product and selectivity: 57-67% ethyl ether and 33-43% ethylene.

Fig. 2 Reaction of Butane over SO4/ZrO2-I(○), SO4/ ZrO2-II(●), and SO4/ZrO2-III(▲): solid lines, at 180℃; dashed line, at 130℃ (pulse reaction conditions)

The SO4/TiO2 catalysts are active for the skeletal isomerizations of butane and isobutane at room temperature13). Butane was converted into isobutane and propane, and the conversion of SO4/ SO4/Fe2O3-I (●), SO4/Fe2O3-II (○), SO4/Fe2O3-III (△), TiO2-I was five times as high as that of SO4/TiO2- SO4/Fe2O3-II at 0℃ (○). II; the maximum activity was observed with calcination at 525℃. The quantity of S was esti- Fig. 1 Reaction of Butane at 25℃ mated by chemical analysis to be 2.11 and 0.01% for the SO4/TiO2-I catalyst calcined at 525℃ and 650℃, respectively12). for butane (Fig. 1), while the catalyst prepared Catalytic activities of the SO4/ZrO2 superacids from Fe(NO3)3 was highest in activity when treated for the reaction of butane were examined, the with 0.25M H2SO4 (Table 1). The catalyst pre- results being shown in Fig. 214),15). The maxi- pared from FeCl3 and 0.5M H2SO4 displayed XRD mum activity was observed with calcination at 625- pattern of a mixture of α-Fe2O3 and Fe2(SO4)3 650℃ for SO4/ZrO2-I, 575℃ for SO4/ZrO2-II, and forms, while the sample similarly prepared from 650℃ for SO4/ZrO2-III; SO4/ZrO2-III was similarly Fe(NO3)3 displayed only the oxide form, without prepared from a commercial hydroxide Zr(OH)4x the sulfate one12). H2O (Nakarai Chemicals, Ltd.). The superacid TGA data of the catalysts treated with sulfate ion of ZrO2 was also prepared from ZrO(CH3COO)2 showed a weight decrease at 550-750℃; this de- (Nakarai Chemicals, Ltd.) as a starting material in crease was caused by decomposition of the sulfate the same manner as that of SO4/ZrO2-II; the maxi- to form SO3, thus the calcination at 500℃ giving mum activity was observed with calcination at the highest activity. The activity and the SO3 650℃12). The quantity of S was estimated to be content were found to depend greatly on the 2.8, 2.2, and 0.2wt% for the SO4/ZrO2-I catalyst calcination temperature of the hydroxide before calcined at 500, 650, and 800℃, respectively16) the treatment. Namely, the catalysts prepared by The reaction of butane was carried out in a pre-calcining Fe(OH)3 [prepared by hydrolysis of recirculation reactor at 25℃ over SO4/ZrO2-I cal- Fe(NO3)3] at 100, 300, and 500℃, then treating cined at 650℃[SO4/ZrO2-I(650℃)]15). Pentane each with 0.25M H2SO4 and finally calcining at and isopentane were observed as products, in 500℃ gave 85, 55, and 11% conversions for the addition to C3 and i-C4. The amount of butane dehydration of 2-propanol and 3.43, 1.78, and measured after 48h, 34.2%, is close to that of the 0wt% for the SO3 contents, respectively. This equilibrium mixture of C4 and i-C4 at 25℃, 27% and hydroxide crystallizes at 430℃, and thus sulfate 73%, respectively. The catalyst becomes colored, ion is not adsorbed on the crystallized oxide, but on to be deactivated when heated in a vacuum at the amorphous or especially hydroxide form to temperatures above 250℃; in particular the Fe2O3 create superacidity10). superacid should be evacuated at temperatures

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below 100℃. The catalysts prepared by heating Zr(OH)4 at 100℃ and 400℃, then treating each with 0.5M H2SO4 and finally calcining at 500℃, gave almost the same conversions for the reaction of butane, while the activities decreased greatly by calcination of Zr(OH)4 over 450℃. Zr(OH)4 crystallizes at 410℃: and thus, the treatment with sulfate ion on the crystallized oxide is not effective, as was observed in the cases of Fe2O3 and TiO216). The catalytic action of TiO2 and ZrO2 materials treated with sulfate ion of different concentrations was examined; the ZrO2 catalysts treated with 0.05 to 0.25M showed the same activities and SO3 contents as those of the sample treated with 0.5M H2SO4, though the Fe2O3 materials showed different activities and SO3-contents. The concentration influenced somewhat the TiO2 catalyst; the

maximum activity and SO3-content were observed (A):Fe2(SO4)3, (B):SO4/Fe2O3-II(500℃). with 0.5-1M12). Fig. 4 XPS Spectra of Fe2O3 Catalysts 2.1.6. Effect of the Preparation Procedures of SO4/ZrO217) Zirconium hydroxide used as a precursor of SO4/ ZrO2 was prepared by hydrolysis of zirconum salts (50-60℃), the final pH being made to vary from 5 by addition of ammonia water until the solution to 9. The solution including the precipitate was attained pH8. An effect of pH of the mother kept in a water bath wormed at 50-60℃ for 2h solution for the precipitated zirconia gel was followed by washing the precipitate 2 times with examined, the results of which are shown in Fig. 3. 0.25l of hot water for each and drying at 100℃. The following two methods were performed. (1) After the zirconia gel prepared above, was sul- 100g of ZrOCl2・8H2O was dissolved in 2l of fated using 0.5M H2SO4, followed by calcination distilled water followed by adding 28% ammonia at 600℃, the skeletal isomerization of pentane was

water dropwise with stirring, and the final pH of performed at 0℃ in a closed recirculation system. the mother solution was made to vary from 4 to 10 As shown in the figure, the conversion into i-C5 by changing the amount of ammonia. The and i-C4 are remarkably dependent on the pH of precipitate was washed 15 times with 1l of water the mother solution. The maximum activity was for each and dried at 100℃. (2) Ammonia water observed at pH 8, and the catalyst prepared by the (28%) was added dropwise into 25g of ZrOCl2・ heating method showed higher activity over the 8H2O dissolved in 0.5l of distilled hot water entire pH range. 2.1.7. Surface Property of SO4/Fe2O3 Experiments using XPS were carried out in order to elucidate the surface property. The spectra of SO4/Fe2O3-II (500℃) are shown in Fig. 4 together with those of Fe2(SO4)3 for comparison. The spectra of Fe 2p1/2, Fe 2p3/2, and O 1s for the former were consistent with those for Fe2O3, 710.8 and 530.2eV for binding energies (B.E.) of Fe 2p3/2 and O 1s, respectively. The SO4/Fe2O3 sample also showed a shoulder peak at 532.1eV, which was similar to that of Fe2(SO4)3, the spectra of Fe 2p3/2 and O 1s for the latter being 711.8 and 531.5eV, respectively. The single peak of S 2p3/2 was observed at 168.4, whose value agreed with that of Fe2(SO4)318). IR spectra of the Fe2O3 superacid and of

Fig. 3 Isomerization of Pentane over Sulfated Zirconia Fe2(SO4)3 for comparison are shown in Fig. 5. at 0℃ The sample showed a spectrum different from that

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into isopentane, isobutane, and others (cracked materials smaller than propane) at 250℃, large amounts of decomposed products being observed. Tin oxide prepared by hydrolysis of tin (II) octylate followed by calcination and the sulfated oxide were highly effective as catalysts for the dehydrogenation of cyclohexanol to cyclohexa- none, more effective than those of the oxides prepared by hydrolysis of SnCl422). In consider- ation of high activity of SO4/TiO2 prepared from Ti(isopropoxide)4 compared with that of the catalyst from TiCl4, more detailed studies of preparation are expected. 2.2.2. SO4/SiO223) Silica gel is obtained by hydrolyzing 100ml of

Fig. 5 IR Spectra of Fe2(SO4)3(A) and SO4/Fe2O3 Si(OC2H5)4 (Wako Pure Chemicals, guaranteed

(500℃)(B) reagent) with 100ml of water and a few drops of HNO3. The mixture is stirred until the gel is formed. The precipitates are obtained by evapo- of iron sulfate; the material showed absorption ration of excess water and ethanol, formed by bands at 980-990, 1040, 1130-1150, and 1210- hydrolysis of Si(OC2H5)4, followed by drying at 1230cm-1, which are assigned to the bidentate 100℃, and powdering. The silica gel (3g) is sulfate coordinated to metal elements7),16),19). It is exposed to SO2Cl2 for 1h followed by evacuating concluded from the results shown in Figs. 4 and 5 HCl evolved by the reaction of surface OH group that the surface is not coincident with Fe2(SO4)3, with SO2Cl2 and excess SO2Cl2 in vacuum, and but composed of Fe2O3 and SO42-12),16). calcining in air at 400℃. 2.2. Preparation of SO4/SnO2, SiO2, Al2O3 The catalyst showed high activity for the de- 2.2.1. SO4/SnO220),21) hydration of ethanol, much higher than that of The SnO2 catalyst with superacidity was synthe- SiO2-Al2O3, the temperature difference between sized by a procedure different from the cases of both catalysts to obtain the same conversions being ZrO2, TiO2, and Fe2O3. Sn(OH)4 is obtained by over 40℃. The maximum activity was observed hydrolyzing 100g of SnCl4・xH2O (Nakarai Chemi- with calcination at 400℃; catalysts calcined at cals, Ltd., guaranteed reagent) dissolved in 2l of temperatures higher than 450℃ were much lower water with ammonia water (28%). Final pH of the in activities. Treatment with H2SO4 and gelation solution should be adjusted to 9.5-10, because the using NH3 instead of HNO3 were not effective to hydroxides are again dissolved at pH values above generate superacidity. It is assumed from DTA 10. The precipitates are washed by decantation and IR experiments that the surface structure is the three to five times until their conversion into col- same as that to be described for other superacids23). loidal state and drying at 100℃. The hydroxides 2.2.3. SO4/Al2O324) (2g) are exposed to 30ml of 3M H2SO4 in a beaker In the case of the sulfate-treated superacids of Fe, for 30min followed by filtering, drying, and Ti, Zr, Hf, Si and Sn, superacid sites are not created calcining in air at 550℃; the maximum activity by the treatment of sulfate ion on the crystallized being observed with calcination at 550℃. oxides but rather on the amorphous forms, Catalysts prepared from the tin hydroxides, followed by calcination to crystallization. The which were obtained from the solution having superacid of Al2O3 is prepared from the crystallized pH7, were much lower in activities than those oxide, γ-Al2O3; highly active catalysts are obtained having pH10. The maximum activity of the by treatment on the crystallized oxide rather than materials treated with sulfate ion of different on the amorphous one. The most active catalyst concentrations was observed with 3M, though the is obtained by exposing γ-Al2O3 to 2.5M H2SO4, material treated with <1M concentration showed followed by calcining in air at 550-650℃. high activity for other superacids. Shown in Table 2 are activities of SO4/Al2O3 The SO4/SnO2 catalyst shows oxidizing action catalysts, calcined at 650℃, for benzoylation of for hydrocarbons at temperatures above 100℃ as toluene with benzoyl chloride, together with their was observed with SO4/Fe2O3. Butane was main- surface areas; the benzoylation, an example of the ly converted into isobutane over SO4/SnO2 at Friedel-Crafts acylation, is generally catalyzed by room temperature, while pentane was converted Lewis acids such as AlCl3 and BF3, and SiO2-Al2O3

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Table 2 Activities of SO4/Al2O3(650℃) Catalysts for the isomerization of butane, the maximum activity Benzoylation of Toluene with Benzoyl Chloride being observed with calcination at 700℃ and and Their Surface Areas treatment with 1M H2SO425). A substance obtained by calcination of Zr(SO4)2 at 725℃ shows an acid strength of-13.16

a) Al2O3 without the sulfate treatment. (Mitsuwa Chemical Co.) at 250℃ followed by powdering below 100 mesh. Calcination is then carried out in air in quartz tubes at 725℃ for 3h is totally inactive. Al2O3-1 and -2 were prepared followed by sealing26). by powdering γ-Al2O3 (Japan Chromato Co., AE- 2.4. Acidity 11) and γ-Al2O3 (KAT6 of Nishio Chemical Acid strength of the superacids was examined by Co.) below 100 mesh, respectively. Al2O3-3 and -4 the visual color change method of the Hammett were JRC-ALO-1 and -3, respectively, supplied indicators, which it may provide a qualitative from Catalysis Society of Japan as reference ranking of catalysts according to their acid catalysts. Al2O3-5 was obtained by hydrolyzing strength, in certain circumstances. The indicator Al(NO3)3 (Wako Pure Chemicals) with aqueous dissolved in solvent was added to the sample in ammonia, washing, drying, calcining the powder form placed in sulfuryl chloride or cyclo- precipitates in air at 700℃ for 3h, and powdering hexane14),20),21); guaranteed grade reagents were below 100 mesh. Al2O3-6 and -7 were obtained by dried over silica gel before use. The indicators thermally decomposing Al(OH)3 and Al(NO3)3, used were m-nitrochlorobenzene (pKa=-13.16), Wako Pure Chemicals, at 700℃ in air for 3h, 2,4-dinitrotoluene (-13.75), 2,4-dinitrofluoroben- respectively. The aluminas (2g) were immersed zene (-14.52), and 1,3,5-trinitrobenzene (-16.04). in 2.5M H2SO4 (30ml) on a filter paper followed The highest acid strength is estimated to be by drying, calcining in Pyrex glass tubes (quartz Ho≦-16.04 for SO4/ZrO2-I calcined at 650℃ tubes for temperatures over 700℃) in air, and [SO4/ZrO2-I(650℃)], SO4/ZrO2-II(575℃), and finally sealing in ampoules until use. SO4/SnO2(550℃) and-16.04

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Table 3 Activities of WO3/ZrO2 Catalysts for the Benzoylation (A) and for the Reaction of Pentane (B)

Table 4 Reactions of Butane and Pentane over WO3/ZrO2 Methanol and air were alternatively passed through the (800℃) catalyst bed under pulse conditions.

Fig. 60 Oxidation of Methanol at 170℃ Cover Supported MoO3 Catalysts (10wt%)

a) C5+i-C5. b)C4+i-C4. hexane into 5.2% propane, 0.4% butane, 0.5%

pentane, and 2.5% isopentane at 50℃ for 24h29),30) In the case of the benzoylation, the effect of lowed by evaporating water at room temperature, modifying the proportion of molybdenum in the drying in air, and calcining in air at 600-1000℃. catalyst was large. The catalysts were prepared by The concentration is 15wt% W based on the the method of evaporation to dryness with 2,5,10 hydroxide, 13wt% W after calcination at 650- and 20wt% Mo based on Zr(OH)4; the maximum 950℃. The analogous superacid is also formed activity was observed with calcination at 600, 650, by the kneading method with tungstic acid 800, and 900℃ for the 20%-, 10%-, 5%-, and 2%-Mo/ (H2WO4) which is insoluble in water; a wet ZrO2 catalysts12). The WO3/ZrO2 superacid with mixture of 10g of Zr(OH)4 and 2.04g of H2WO4 the highest acidity was shown to contain (Nakarai Chemicals, Ltd.) with a little water is 13wt% W, whose value was estimated to be the kneaded for 3h27),28). amount of tungsten that strongly interacts with Ten grams of Zr(OH)4 are impregnated with ZrO2 in monolayer. The 13wt% W is equivalent 2.54g of molybdic acid (H2MoO4) (Nakarai Che- to 6.6wt% Mo. Sublimate based on an excess of micals, Ltd.) dissolved in 2ml of ammonia water molybdenum oxide was observed with calcination (28%) and 15ml of water followed by evaporating of the 20%- and 10%-Mo/ZrO2 catalysts; for this water at room temperature, drying, and calcining reason the calcination temperatures showing the in air. The concentration is 5wt% Mo metal maxima in activity seem to be much lower than based on the hydroxide29). those for the 2%- and 5%-Mo/ZrO2 catalysts. 3.2. Catalytic Activities Molybdenum oxide is well known as an oxi- The WO3/ZrO2 catalysts were quite effective for dation catalysts, thus the present substance would the benzoylation of toluene with benzoic anhy- be an oxidation catalyst with superacidity, of dride at 80℃, and for the reaction of pentane at which new catalytic-action is expected. The 280℃, as shown in Table 3. The maximum MoO3/ZrO2 catalyst was examined in the oxida- activity was observed with calcination at the tion of methanol to formaldehyde; the results are surprisingly high temperatures of 800-850℃, for shown in Fig. 6, together with those of molyb- both reactions. The SiO2-Al2O3 catalyst was denum oxides supported on Al2O3, TiO2, and SiO2, totally inactive for both reactions. The catalyst prepared in the same manner as the ZrO2 catalyst. treated at 800℃[WO3/ZrO2(800℃)] was active for It is seen that ZrO2 is most effective, and molyb- isomerization of butane at 50℃ and for pentane at denum oxide prepared by calcining H2MoO4 at 30℃, as shown in Table 427),28),30). 500-700℃ is inactive30) The MoO3/ZrO2 catalysts were also effective for 3.3. Properties the benzoylation; high activities were observed The acid streng th of WO3/ZrO2(800℃) was upon calcination at 750-800℃. The catalyst estimated to be Ho≦-14.52 by a color change calcined at 800℃ [MoO3/ZrO2(800℃)] converted method using Hammett indicators. The MoO3/

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when a tetragonal system is formed. In fact, appearance of the WO3/ZrO2 catalyst differs greatly from the inactive material prepared from the crystallized oxide. The former is white in color, WO3 being highly dispersed into the ZrO2 lattice, whereas the latter is not, the color being close to that of WO3 itself12). Specific surface areas of the WO3 and MoO3 catalysts showed that the areas of both catalysts are much greater than those of the oxides without tungsten and molybdenum oxides as was also observed with the sulfate superacids, especially in

(A): Crystallization temperature of ZrO2 determined by the case of calcination at 800℃27),30) DTA. 3.4. WO3/SnO2, WO3/TiO2, WO3/Fe2O331),32) Sn(OH)4, H4TiO4, and Fe(OH)3 were obtained Fig. 7 Relation between Calcination Temperatures of by hydrolyzing guaranteed grade reagents of SnCl4, Zr(OH)4 before Treatment with Tungstate and Their Catalytic Activities for Pentane (○) and TiCl4, and Fe(NO3)3, respectively, with aqueous Hexane (●) ammonia, washing, drying at 300℃, and powdering the precipitates (32-60 mesh). The hydroxides were impregnated with aqueous ammonium ZrO2(800℃) catalyst itself was colored (yellowish metatungstate [(NH4)6(H2W12O4O), Nippon In- green), and thus the acid strength was not organic Colour & Chemical Co.] followed by estimated by the visual color change method. It is evaporating water, drying, calcining in air for 3h. considered that the latter catalyst bears surface The concentration was 15wt% W based on the acidity higher than Ho=-12.70 (ca.-13), judging hydroxides (11-13wt% after calcination). from the results of reaction. The catalysts were examined in reaction of The relation between the precalcination tem- isopentane; the maximum activity was observed perature of Zr(OH)4, before the impregnation, and with calcination at 1000℃ for WO3/SnO2 and the catalytic activities for reactions of pentane and 700℃ for WO3/TiO2 and WO3/Fe2O3. Their hexane was examined, and the results are shown acid strength was estimated to be Ho=-13--14, in Fig. 727). The catalysts prepared by heating --13, and -13--12, respectively. Zr(OH)4 at 100-300℃, then impregnating each 3.5. B2O3/ZrO233),34) with the tungstate and finally calcining at 800℃ Zirconium hydroxide was impregnated with gave almost the same activities, while the activities aqueous boric acid followed by evaporating water decreased greatly by calcination of Zr(OH)4 at tem- and calcining in air at 500-700℃ (3-7wt%B). peratures of over 400℃, the crystallization tem- The same catalyst was obtained by suspending the peraure of ZrO2. hydroxide in 2-propanol solution of trimethyl XRD measurement of the catalysts was borate followed by adding water to hydrolysis of performed27),29). WO3/ZrO2(800℃) and MoO3/ the borate. ZrO2(800℃), whose activities were highest, gave The highest acid strength was estimated to be 100% tetragonal form, while WO3/ZrO2(1000℃), Ho=-13. whose activity was quite low, showed the pattern to be monoclinic system, in addition to the crys- 4. Conclusion tallized WO3. The catalyst prepared by precal- cining Zr(OH)4 at 700℃ to ZrO2, followed by The author has attempted to present the recent impregnating with the tungstate and calcining at works on syntheses of solid superacids. Sulfate- 700℃, showed the pattern almost coincident with supported metal oxides were stable attributed to that of WO3/ZrO2 (1000℃), and the similarly heat-treatment at high temperatures for the prepared MoO3/ZrO2, which were not active at all preparation, but it is still hoped to synthesize for the benzoylation, was also monoclinic. superacids with the system of metal oxides. Therefore, it is concluded that superacid sites are Tungsten or molybdenum oxide supported on not created by impregnation on the crystallized oxides of Zr, Sn, Ti, and Fe were prepared by a new oxide, but on the amorphous form whose cal- procedure, their stability being satisfactory so far. cination then converts to the crystalline form; i.e. It is hoped that the procedure will be extensively tungsten or molybdenum oxide combines with applied to other metal oxides for new solid zirconium oxide to create superacid sites at the time superacids.

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References Am. Chem. Soc., 79, 4904 (1957). 20) Matsuhashi, H., Hino, M., Arata, K., Chem. Lett., 1988, 1) Gillespie, R. J., Acc. Chem. Res, 1, 202 (1968). 1027. 2) Gillespie, R. J., Peel, T. E., Adv. Phys. Org. Chem., 9, 1 21) Matsuhashi, H., Hino, M., Arata, K., Appl. Catal., 59, 205 (1972). (1990). 3) Arata, K., Hino, M., Hyomen, 19, 75 (1981). 22) Hino, M., Arata, K., Chem. Lett., 1990, 1737. 4) Hino, M., Arata, K., Hyomen, 28, 481 (1990). 23) Matsuhashi, H., Hino, M., Arata, K., Catal. Lett., 8, 269 5) Arata, K., Shokubai, 31, 512 (1989). (1991). 6) Arata, K., Adv. Catal., 37, 165 (1990). 24) Arata, K., Hino, M., Appl. Catal., 59, 197 (1990). 7) Arata, K., Hino, M., Mater. Chem. Phys., 26, 213 (1990). 25) Arata, K., Hino, M., React. Kinet. Catal. Lett., 25, 143 (1984). 8) Arata, K., Hino, M., Shokubai, 24, 241 (1982). 26) Arata, K., Hino, M., Yamagata, N., Bull. Chem. Soc. Jpn., 9) Arata, K., "Shokubai Kohza," ed. by Catal. Soc. Japan, 63, 244 (1990). Kodansha Scientific, Tokyo (1986), Vol. 11, p. 31. 27) Arata, K., Hino, M., Proc. 9th Int. Cong. on Catal., 10) Hino, M., Arata, K., Chem. Lett., 1979, 477. Calgary, Canada, 1988, p. 1727. 11) Hino, M., Arata, K., Chem. Lett., 1979, 1259. 28) Hino, M., Arata, K., J. Chem. Soc., Chem. Commun., 1988, 12) Hino, M., Arata, K., unpublished results. 1259. 13) Hino, M., Arata, K., J. Chem. Soc., Chem. Commun., 1979. 29) Hino, M., Arata, K., Chem. Lett., 1989, 971. 1148. 30) Arata, K., Hino, M., Shokubai, 31, 477 (1989). 14) Hino, M., Kobayashi, S., Arata, K., J. Am. Chem. Soc., 101, 31) Arata, K., Hino, M., Proc. 10th Int. Congr. on Catal., 6439 (1979). Budapest 1992, eds. by Guczi, L., Solymosi, F., Tetenyi, P., 15) Hino, M., Arata, K., J. Chem. Soc., Chem. Commun., 1980, Elsevier, Amsterdam (1993), p. 2613. 851. 32) Hino, M., Arata, K., Bull. Chem. Soc. Jpn., 67, 1472 (1994). 16) Hino, M., Ph. D. Thesis, Hokkaido University, Hokkaido, 33) Matsuhashi, H., Kato, K., Arata, K., "Acid-Base Catalysis Japan, 1982. II," eds. by Hattori, H., Misono, M., Ono, Y., Kodansha, 17) Tatsumi, T., Matsuhashi, H., Arata, K., 43rd National Tokyo (1994); Stud. Surf. Sci. Catal., 90, 251 (1994). Meeting of the Petroleum Society of Japan, Sapporo, 34) Hino, M., Arata, K., 70th National Meeting of the October 1994, Abstr. No. 46. Catalysis Society of Japan, Niigata, October 1992, Abstr. 18) Yabe, K., Arata, K., Toyoshima, I., J. Catal., 57, 231 (1979). No. 4F409. 19) Nakamoto, K., Fujita, J., Tanaka, S., Kobayashi, M., J.

要旨

固体超強酸触媒の調製

荒田一志

北海道教育大学函館校自然科, 040北海道函館市八幡町1-2

固体超強酸触媒の調製に関する最近の研究である, 硫酸-金成される。すなわち, 水溶性のメタタングステン酸アンモニウ属酸化物とタングステン-, モリブデン-, およびボロン-金属ムまたはアンモニア水に溶解されたモリブデン酸を無定型の酸化物についてまとめた。硫酸イオンをFe, Ti, Zr, Hf, Sn, ZrO2に含浸した後, 空気中800～850℃で焼成する。同様のおよびSiの無定型酸化物に吸着させて空気中500℃以上で焼超強酸がメタタングステン酸アンモニウムをSn, Tiおよび成すると,Ho≦-16.04までの酸強度をもつ超強酸が得られる。 Feの無定型酸化物に含浸した後空気中焼成しても調製され, Al2O3の超強酸は結晶型の酸化物より得られる。イソペンタンの反応における最高活性はSnの場合で1000℃, 硫酸による超強酸の調製法と同様の方法により, WO3(13 TiおよびFeのもので700℃焼成で示された。ジルコニアに wt%)またはMoO3(6wt%)をZrO2に担持することにより, ボロンを担持しても超強酸性を示すが, 強度は弱い。 Ho≦-14.52までの酸強度をもつ金属酸化物による超強酸が合

Keywords Superacid, Solid acid catalyst, Sulfated metal oxide, Catalyst preparation, Tungsten oxide, Zirconium oxide

石油学会誌 Sekiyu Gakkaishi, Vol. 39, No. 3, 1996